1. Field of the Invention
The present invention relates to an imageable element having an imaging layer comprising aromatic diazonium salt containing compounds and a method of producing the same. More particularly, the present invention relates to an imageable element, which comprises an aromatic diazonium salt containing compound having an alkoxy substituent and an aromatic diazonium salt containing compound free of an alkoxy substituent.
2. Description of the Prior Art
Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. In the art of photolithography, a photographic material is made imagewise receptive to oily inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Negative-working coatings for that purpose include light-sensitive polymer layers containing diazonium salt compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers, particularly diazonium salt condensates, which are widely used. Upon image-wise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the photosensitive composition from the unexposed areas.
Common difficulties encountered by practitioners of lithography include the poor solubility of the layer components in the solvents selected for the preparation of the coating solutions, insufficient differentiation between the image areas and the non-image areas, poor developability and resulting undesirable scumming during printing, low practical light-sensitivity, poor adhesion of the imaging layer, including low resistance to delamination under humid conditions, poor resistance to press chemicals and short press life.
U.S. Pat. Nos. 3,867,147 and 3,849,392, disclose synthesis of mixtures of diazonium salt condensates and copolymers, which contain both methoxy- and non-methoxy-diazonium salt groups. Polyvinyl acetal binders, having acid groups, are also disclosed. However, this patent does not disclose mixtures of methoxy- and non-methoxy-diazonium salt condensates, nor does it disclose the effect of the condensates on presslife and resistance to press chemicals.
U.S. Pat. No. 4,533,620 discloses copolymers which contain both methoxy- and non-methoxy-diazonium salt groups. It does not disclose mixtures of the corresponding methoxy- and non-methoxy-diazonium salt condensates.
European Patent Application No. 896,251 discloses a method and apparatus for making a lithographic plate in which, following imaging, a single developing fluid is utilized. The examples utilize mixtures of methoxy- and non-methoxy-diazonium salt condensates. However, the weight ratio of methoxy- to non-methoxy-diazonium salt condensates is less than one, corresponding to a calculated molar ratio of less than 0.9 with the same anions. The disclosed binders include polymethyl methacrylate dispersion in 98% hydrolyzed polyvinyl acetate and polyvinyl alcohol.
European Patent Application No. 902,325 claims a mixture of two diazo resins having an inorganic acid anion as the counter anion for one of the diazo resins and an aliphatic or aromatic organic acid anion for the other. The use of a polyurethane binder is also disclosed. One of the examples utilizes a mixture of methoxy- and non-methoxy-diazo resin with a methoxy to non-methoxy diazo resin weight ratio of 0.43:1, which corresponds to a calculated molar ratio of 0.3:1.
U.S. Pat. Nos. 5,846,685 and 4,408,532 disclose light sensitive compositions and lithoplate compositions based on diazonium salt condensates, including binders and substrates.
None of the above references discloses an imaging layer having an aromatic diazonium salt containing compound having an alkoxy substituent and an aromatic diazonium salt containing compound free of an alkoxy substituent.
Accordingly, it is an object of the present invention to provide an imaging layer based on a mixture of an aromatic diazonium salt containing compound having an alkoxy substituent and an aromatic diazonium salt containing compound free of an alkoxy substituent, which does not have the above-mentioned drawbacks and which has good storage properties, superior press life and resistance to plate chemicals.